An Introduction to Particle Dark Matter^†

Round 1

Reviewer 1 Report

The review titled ‘An introduction to particle dark matter’ presents some important (possible) features of dark matter particles, gives the whole physical pictures of the role of dark matter in the universe and their relationships with the structure formation of the universe. Most interesting, the authors, with their own understanding, show how to find dark matter and where to search for them, from the point of view of a particle.

This paper, which is not a survey of dark matter evidence or models, will undoubtedly trigger new ideas of young students and scholars who are interesting in the study of particle dark matter. This paper is deserved for publication.   

Author Response

We thank Referee for the kind assessment of our work and its usefulness.

Reviewer 2 Report

This manuscript represents a collection of notes based on lectures given by S. Profumo in a
School on Cosmology in September of 2016. My impression from the notes is that they provide
a perspective on different particle dark matter models (circa 2016), their production mechanisms,
and detection possibilities. The notes also present a few of the most prominent examples where
current anomalies in observations might be connected to dark matter detection.

Overall, I think the notes are relevant since they aim at providing a synthesis of all
these different aspects of particle dark matter. However, in my opinion the manuscript requires
additional work in order to transform these notes into a more cohesive manuscript that can be
read as a single work and that it includes appropriate references for a review. The extent to which
this should be done depends both on the goal of the authors in publishing this work and the scope
that the editors of Universe have in mind for a review article. In the following, I give more
details on my main concerns followed by minor comments.

Main comments.

1) References

The manuscript has a very low number of references (only 6 quoted in the bibliography and several
more scattered in footnotes, the few references that appear are not listed in order). In my perspective
a review article on the topic needs to include a number of key references, which in this case are many given
the scope of the manuscript. These references are currently missing and the few present are scattered
and selected in a seemingly arbitrary way.

This is a critical point that needs to be addressed before considering further this manuscript for publication.

2) Cohesion, consistency and revision of equations

Although the manuscript does have a well organized structure, it needs to be revised and polished in a major way.
The addition of an index will also help in this regard.

In many of the equations the different variables are not defined (e.g. eq. 95,101), and some equations are wrong (e.g.
eq. 63).

Several concepts/equations are mentioned but not properly defined or described at all. There are many examples of these,
in the following, I will list the most relevant I found.

- the J-factors introduced in page 17 (not defined, and uncertainties not commented on: based on incomplete DM distribution).

- page 2: it is mentioned that modified gravity does not work because "the Universe is highly non-linear". This is
not obvious at all and the context doesn't provide enought evidence to support this. This needs to be expanded or
removed.

- page 2: Quoting:

"...which is a cross section typical of the strong interaction. Is this small? Well... Also, if the cross
section is slightly smaller, then there are no visible effect. On the other hand, if it is slightly
larger it would be a disaster. Is “collisional” self-interacting DM a “natural” possibility?..."

The wording is perhaps suitable for a lecture but I think is not appropriate for a review article. Mor importantly,
I'm not sure 'naturalness' can be properly defined for the DM problem given the large range of classes/types of allowed
particle DM models. It is also imprecise to say that if the cross section is less than 1 cm^2/gr there is no
visible effect, since this is only true below 0.1 cm^2/gr. It is inaccurate to say that if the cross section is
"slightly" larger it would be a disaster. There is no reason or explanation given for this, but I assume it
refers to constraints on cluster scales (e.g. the Bullet Cluster), it is of course well known that many
popular self-interacting DM models are velocity dependent (referred to in Section 4.3), which easily avoid this
issue since the constraints at galactic scales are much weaker. Overall, SIDM models are much less "fine-tuned"
that implied by the authors here.

- page 3: quoting:

Exercise 6 could be incorporated as a part of the discussion here since otherwise it seems that there is no relevant
room for quantum effects, or at least this is not expanded at all, which is important given the body of research
in "fuzzy DM". This is an avenue that is clearly very active.

- page 3:

Regarding the issue of DM being a fluid, I don't understand the explanation. I guess the point is whether or not DM
can be made of compact/solid massive objects? Describing it in this way is perhaps more appropriate since the concept of
fluid can be confusing in this sense, e.g. collisionless CDM cannot be described as a fluid in the standard
thermodynamic sense...

- page 8: Section 2.3 seems completely out of place. I suggest removing it, or alternatively, it needs a better
Introduction to emphasize that is possible to have models where is not just the interactions that can be different,
but where the expansion history can be different.

3) Excercises

The notes include a few exercises, which I assume where part of the School lectures. I think these are
not necessary for the review. If they were more and connected to each subsection, they could be quite a
nice addition as an appendix, but as they are, they seem arbitrarily picked.

4) Introduction

Most of the points in the comment 2) above are for the Introduction. I like that is presented as a list of the
microscopic properties of DM, but it needs to be expanded and revised completely. At the end of it I would recommend
to add that in interactions, weak scale interactions with the SM are also possible: most of the article focuses on
WIMPs and yet these are not mentioned in the Introduction. I would also recommend to quote at some point the current
constraints on these interactions from the different classes of experiments: direct detection, indirect detection
and colliders. A discussion on the stauts of these and their implication (or fuuture implication) for the relevance
of WIMPs and the "WIMP miracle" is perhaps timely.

5) Abstract

The current abstract is not appropriate.

Minor points:

-page 2: Is DM actually dark? A fraction of it could be highly dissipative actually (e.g. the work of Lisa Randall's
group)

- typo in page 2 before eq 5, it should say: larger than the typical size of a cluster

- typos in eq. 5, the signs in the equations should be reversed

- typo in eq. 21

- page 5 towards the end (just before eq. 27): it is important to emphasize that WIMPs (at least thermal relic WIMPs)
are assumed to be symmetric. This is implicit before eq. 23, but it needs to be highlighted as a key feature of
vanilla WIMPs

- page 6 after eq. 36: the quoted mass bound is valid only for thermal WIMPs

- page 7: eq. 39, whit is only valid without Sommerfeld enhancement

- page 8: typo in eq. 52

- page 9: freeze-out and chemical decoupling should be defined as the same thing at some point

- page 9, eqs. 59 and 60 should have a different cross section

- page 9, eq. 60: I think there is a typo in eq. 60, it should be n_non-rel

- page 9, eq. 63 has a clear typo

- page 11: the title of the subsection 3.1 should be moved at the beginning of this page, since it is here
where the discussion on direct detection starts

-page 11: eq 73, m_N not defined

- page 11: eq 74, it should be clarified that this is for 1 kg of material

- page 13 line 168: decay rate -> decay/annihilation rate

- page 14 eq 95, some variables not defined

- page 14, before eq. 96, the values of R and h are for the Milky Way disk

- page 15 eq 101, some variables not defined

- page 15, after eq. 108, the pulsar interpretation is not introduced properly (e.g. it should be introduced after
eq 105), PSR is not defined.

- page 16 eq 113, E^\odot not defined

- page 20 eq 134-135, some variables not defined

- page 25 after eq. 184: "could explain high velocity pulsars", no context, no explanation

-page 26, line 390-391: Why the simplest sterile neutrino does not work? Ref.?

Author Response

Author Response File: Author Response.pdf

Reviewer 3 Report

The article aims at presenting an introduction to particle dark matter. However, it is difficult to see who would find the article useful, as it clearly is not a review (the article is not exhaustive, it is poorly referenced, the reference style changes in the middle of the article and the references are not well organised) as the authors also state in the Introduction but the main problem is that it is not a good introduction to the topic either. 

While the content itself is probably correct, the target level and quality of writing vary considerably from one section to another, way too many things are left unexplained and/or unreferenced and symbols undefined, and as a result the notes do not form a coherent and comprehensible introduction to the topic. The article is merely a collection of unorganised material, and while it may work as informal lecture notes, it is not suitable for publication in its present form.

The shortcomings are so numerous that I do not believe anything but very excessive re-writing would improve it, and even if that was done, I do not see what value the article adds to e.g. these recent introductions to particle dark matter: 1705.01987, 1812.02029, 1904.07915. I therefore recommend rejecting the article.

Author Response

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The authors have improved substantially the manuscript addressing all my major concerns. I would therefore recommend this paper for publication in Universe. 

Before publication, I would recommend a final complete revision to correct the many, albeit minor, grammar issues and typos that still remain in the paper.

Author Response

Dear Editor,
We are indebted to the referees, for having dedicated so much of their
time to our manuscript and for their extensive and thorough revision. We
have revised the text, from the linguistic viewpoint, stylistic one and also
about the content, taking into account all the comments made. We hope
now the paper is suitable for publication.

Author Response File: Author Response.pdf

Reviewer 3 Report

The authors have made substantial improvements to the manuscript. In particular, they have added content, revised the presentation of material, and added references.

Yet, the article is not a good review nor a good introduction to the topic. In particular, it fails most badly in what it primarily tries to be: an introduction to particle dark matter. The authors seem to assume the reader is very familiar with topics like general relativity, cosmology, statistical mechanics, advanced quantum field theory and particle astrophysics, and therefore familiar with most of the concepts and formulae presented in the article, as so much is left unexplained, undefined, and unreferenced. Also, the presentation of material is in most parts rather unbalanced and unorganised, and I'm afraid most readers – especially students – will find the notes nothing but confusing.

The authors should ask themselves the following questions:

1. What is the target audience of this article?

2. What are the prerequisites to understand the presented material?

3. What should one learn by reading this article?

It would be good if the authors not only answered these questions themselves but also clearly spelled them out in e.g. their introduction.

Had this been the first version submitted to peer-review, I would still have written the very same report as during the first round. In particular, while most of the content is probably correct, the target level and quality of writing vary considerably from one section to another, way too many things are left unexplained and/or unreferenced and symbols undefined, and as a result the notes do not form a coherent and comprehensible introduction to the topic.

To elaborate on why I think this is the case, let me point out a few shortcomings in the manuscript:

- Introduction has to be improved substantially. In its present form, it does not exhibit professional, high quality writing style. At the moment it must be very confusing to a reader who does not know anything about dark matter. For instance, before the authors mention dark matter, they begin by discussing dark energy. It is confusing why they say that one can measure the dark energy contribution to the total energy density, without ever saying what that contribution is and/or how it relates to dark matter.

- Define H_0, G_N in (1). While I think this particular point is not a very big flaw, the number of undefined symbols is, in general, one of the biggest shortcomings of the article. Most symbols are not defined, whereas I think the authors should clearly spell out in words what different symbols mean and what the physical concepts behind them are.

- Mass of the Sun below (3) is wrong.

- On page 3, the authors mention that over-densities in the linear regime grow linearly with the scale factor. This is only true during a matter-dominated epoch and in a suitable gauge, and the authors should mention this.

- When discussing how dark DM is, it does not help much if the authors just say that it is “believed that it is not bright”. One should define what this means. Also, I have never seen the acronym PIDM used for “Partially Interacting DM” but “Planckian Interacting DM”, see e.g. https://arxiv.org/abs/1709.09688

- On page 3, \lambda is used in two consecutive sections for two different quantities. This should be changed. Also, define h and S above (6).

- Not sure what “teach us how much important could be quantum effect” or “interactions are not too much bright” mean on page 4. The article contains numerous non-grammatical expressions and the authors should correct them.

- Define “coarse graining” and “impulse approximation” on page 4, clarify “binding energy [of X]”.

- On page 4, the authors define SM to refer to the Standard Model. On page 5, they define it to mean statistical mechanics. This should be avoided. Also, define “GR” on page 5. Correct “Plank”.

- Define \rho^0_today in (20). Correct the text below (21), in particular “freeze-out and chemical decoupling should be defined as the same thing at some point”. Explain what \Lambda_QCD is.

- At least two different symbols are used for the critical density; compare e.g. (27) and (1), and note the typo in (21). Define E_EW in (32).

- Give references to the statement below (35). Explain how the unitarity limit (38) is derived.

- Explain how the integrated collision term (48) is derived or give references.

- Define P_\phi in (54).

- I find the discussion on quintessence somewhat confusing. For example, the authors derive the density parameter “of the quintessence field” and show in (57) that this fluid's energy density can easily dominate the density of the DM component in the standard calculation. Is this correct? By “the density parameter of the quintessence field”, do the authors mean “the DM density parameter in the quintessence scenario”?

- I do not understand the sentence above (60). Also, clarify why many collisions are needed. Also, explain why N = (p/\delta p)^2 in (61).

- Clarify the statement above (66) that structures can only collapse when T\sim m_\nu. Why is that the case? Also, primordial black holes and mini-/microhalos provide a counter-example to this.

- Define M_{Ly-ff} in (68). Define E_\nu in (70). Clarify the sentence above (71), which says ”The maximal recoil momentum [of a nucleus] for a DM particle...”

- Specify what is meant by abundances below (74). Also, what is “1400 thousands of rock”?

- Define all symbols in (75)-(88). In its present form, this section is incomprehensible to a reader who does not already know what is being done here.

- Give references that support the claims below (89).

- Define \Delta\psi in (91). Define “Fermi mechanism” above (96).

- Define “secondary-to-primary ratio” above (100). Clarify “looks very good” below (100).

… and so on. As said, the fact that symbols are left undefined throughout the article is a major problem, and so is the fact that sometimes different symbols are used for the same quantities.

Here are also some other suggestions for improvements:

- I find it odd that the abstract first states what the article is not. I suggest moving the first two sentences after the third one.

- It would be easier if the references were organised in the order of appearance.

- I suggest moving the part “For example, a smoking gun signal is a monochromatic, spatially extended gamma-ray emission. Indeed, a monochromatic gamma-ray line with a diffuse morphology has no astrophysical counterparts.” that now appears in the end of Sec. 3.2.2. to the beginning of 3.2.1 to give motivation for this type of searches and discussion on them.

- The Tremaine-Gunn bound discussed on page 4 quotes a number different from the one given below (169). This should be corrected. See also https://arxiv.org/abs/0808.3902

- The authors never properly explain freeze-in. Also, they  should clarify what they think is the difference between “freeze-in” and “non-thermal production”.

- Below (181), the authors give a “summary” of the discussion on sterile neutrinos but they also point out things that were not discussed in the section above, for example baryogenesis. This does not exhibit good writing style.

- The material on the 3.5 keV line is very unbalanced with the rest of the article. Also, I find it unprofessional to add a section to this kind of review/introduction to shamelessly promote an article by one of the authors, as currently done in the end of Section 3.

Overall, the article's shortcomings are still so numerous that I do not feel it is reasonable to continue the process. If the authors want the notes published somewhere, they should put considerable effort in improving their content and not submit half-baked sketches to peer-review. It is primarily the authors' task to do exhaustive proof-reading and put serious effort in improving the overall quality, not the referees', at least not to this extent.

In its present form, the manuscript makes me wonder if these are just hastily written lecture notes which the authors want to get published to boost their publication record. I also still do not see what extra value this article would give to the community as a whole, given the existence of multiple very good review and introductory articles on the topic, for example those cited in my previous report. The authors should read them and seriously think about how to improve the quality of their article accordingly.

Due to the above reasons, I strongly recommend rejecting the article, and not considering it for publication in Universe again.

Author Response

Dear Editor,
We are indebted to the referees, for having dedicated so much of their
time to our manuscript and for their extensive and thorough revision. We
have revised the text, from the linguistic viewpoint, stylistic one and also
about the content, taking into account all the comments made. We hope
now the paper is suitable for publication.

Author Response File: Author Response.pdf